Medical devices are implanted in humans or animals for many reasons. Some of these devices are used to monitor one or more bodily functions. Other devices are used to stimulate or out rightly control bodily functions. Often, the medical devices will include some kind of communications circuit for receiving signals used to power and/or control the devices, or for sending outside a patient's body information about the medical device or bodily functions monitored or controlled by the device. Typically, medical devices are powered by an electric power supply, such as a battery, that provides the voltage and current needed for their operation.
Medical devices are often intended to be implanted in a patient's body for many years, and in some instances, for the rest of a patient's life. As such, the power supplies used to power these long-term medical devices are implanted in a patient at a location that permit easy access from outside the patient's body for recharging or replacement of the power supply. Typically, these power supplies are recharged by energy drawn from an alternating magnetic field generated outside of a patient's body and extending into inside of the patient's body using a pair of coils. The pair of coils includes a first or primary coil that generates the alternating magnetic field when an electric current is flowing therein and a second or secondary coil that is implanted in a patient's body. Alternatively, the second coil implanted in a patient's body may be connected directly to a power supply or to a medical device implanted in the patient. There can exist a problem that it is difficult to transfer sufficient amounts of energy for driving such a medical device. Thus, it would be desirable to provide a coil system including primary and secondary coils allowing a relatively high rate of energy transfer.